There are lifting jacks which have a compressed air drive (referred to as climbing jacks). There are also lifting jacks with a spindle drive. There are also lifting jacks in which a hydraulic ram is used in the lifting device.
All these lifting jacks have an upwardly and downwardly movable carriage which supports the lifting tackle. The carriage runs up and down a column. There is an operating switch for "lift" and another switch for "lower".
Such lifting jacks are not used just singly but in pairs, in two pairs, in three pairs, etc.
An ideal situation would be for all the carriages always to be at the desired starting point. The simplest case is where all carriages are at the same absolute height. In such an instance, for example, an omnibus will be lifted vertically upwardly.
There is however also a case where, for example, one of the lifting jacks is on ground which is lower than the others. This means that its carriage must be higher so that the carriages as a whole are once again at the same level.
In practice, however, there are always cases where, although all lifting jacks are standing on ground which is level, the carriage of one lifting jack (or the carriages of several lifting jacks) are at different heights from one another. This happens when an object is intentionally lifted at an angle or when an object has different projections which require the lifting tackle to apply the carriage at a different height.
However, it is necessary always to ensure that the desired height disposition of the carriages is retained.
Many systems work on the basis whereby all the carriages move to the lowest position at some time or another and then adopt this position as a reference position. However, with such a system, the carriages run away if there is frequent upwards and downwards movement and if the reference position is not achieved for any length of time. For various reasons, the carriages run at different speeds even with careful attunement, maybe because there are structural tolerances and maybe because the load on one carriage is greater than on the other, etc.
In the case of motor-driven spindle drives, one solution is for the electric motors to be so heavily over-dimensioned that friction is negligible and if it is then assumed that the spindle nut will after a specific period be dependably at a specific location. In this case, over-dimensioning is a disadvantage. Furthermore, these motors are mostly provided at the upper end of the column so that the lifting jacks become top heavy.
All systems which are driven without clutches also have the disadvantage that the motors and the gearboxes which are always on the downstream side constitute a considerable mass, so that they neither start up at the same time when swtiched on nor stop at the same time when switched off. This, too, is a reason for a lack of synchronism.
There are also highly electronic devices which operate digitally, count spindle revolutions, calculate paths, etc. For rough workshop operation of portable lifting jacks, it is true that very intelligent but not sufficiently rugged systems are inappropriate.
Where such lifting jacks are concerned, a certain lack of synchronism can be overcome because they move loads in the range from 3 to 15 tons. In bulk, such loads are so large that a lack of synchronism is hardly noticeable. In practice, it has been demonstrated that with the distances which arise between one lifting jack and the next, and with the loads which are involved, a 5 cm lack of synchronism over the group is, by and large, tolerable.